lib: add setsockopt IP_TRANSPARENT

Add sockopt_ip_transparent(), a thin wrapper around
setsockopt(sock, SOL_IP, IP_TRANSPARENT) guarded by #ifdef IP_TRANSPARENT.

This lets daemons such as bgpd create transparent sockets when
running on kernels that support the option, while keeping the build
portable on systems that do not provide it.

Signed-off-by: Vincent Jardin <[email protected]>

Author: vjardin

lib/sockopt.c

@@ -739,3 +739,21 @@ int setsockopt_tcp_keepalive(int sock, uint16_t keepalive_idle,
 	return 0;
 #endif
 }
+
+/* set IP_TRANSPARENT to socket */
+void sockopt_ip_transparent(int sock)
+{
+	int err = -1;
+
+#if defined(IP_TRANSPARENT)
+	const char flag_on = 1;
+
+	err = setsockopt(sock, SOL_IP, IP_TRANSPARENT, &flag_on, sizeof(flag_on));
+#else
+	err = ENOPROTOOPT;
+#endif
+	if (err < 0) {
+		flog_err_sys(EC_LIB_SYSTEM_CALL, "%s failed: setsockopt(%d, IP_TRANSPARENT): %s",
+			     __func__, sock, safe_strerror(errno));
+	}
+}

lib/sockopt.h

@@ -170,6 +170,14 @@ extern int setsockopt_tcp_keepalive(int sock, uint16_t keepalive_idle,
 				    uint16_t keepalive_intvl,
 				    uint16_t keepalive_probes);
 
+/*
+ * Set IP_TRANSPARENT option to socket
+ *
+ * sock
+ *    Socket to enable option on.
+ */
+extern void sockopt_ip_transparent(int sock);
+
 #ifdef __cplusplus
 }
 #endif